Museum of Lost Effects: Interfax Harmonic Percolator

Few guitar pedals can rival the cult cachet of the Harmonic Percolator, a singularly ugly distortion stompbox produced in minuscule numbers in the early ’70s by Interfax, a small company based in Milwaukee, Wisconsin. And guess what? They sound ugly too, though they’re ugly in a cool and useful way.

They don’t get much rarer — or uglier. (This is a cosmetically faithful reproduction from Theremaniacs.)

I’ve been wanting to write about these for years, but was hindered by the fact that I don’t have access to one. No one does! Well, except the pedal’s best-known user, producer/guitarist Steve Albini. (Steve has posted several popular YouTube videos in which he sings the praises of the original and evaluates it against modern clones.)

But I revisited the idea recently when Christian Magee, who runs Tube Depot, sent me a couple of old 2N404A transistors from a stash he recently acquired. This rare PNP germanium transistor appeared in the original, along with an NPN 2N3565 (also rare, but not as ridiculously rare as the 2N404A). Yes—this pedal uses both a positive-ground germanium transistor and a negative-ground silicon transistor in the same circuit. (Another Fuzz Face/Tone Bender clone, this ain’t!)

I’ve always found this circuit compelling, and not just because of its weird architecture. It truly has a unique distortion character. Chuck Collins, who makes the cosmetically faithful clone pictured above, talks about the pedal in Tom Hughes’ Analog Man’s Guide to Vintage Effects:

But it was only a matter of time for someone to notice that this pedal does the same thing that tube-type amplifiers do; that is to create even-order harmonics when pushed into saturation or overdrive.

Man, I do not hear what Collins hears! To my ear, the distortion is much more akin to solid-state distortion, like overdriving a tape recorder input. Fortunately, most of us have matured beyond old-school distortion sensibilities (“Tube distortion good, all other distortion bad!”), and many players can put this razor-edged timbre to good use. I totally get why Albini digs it—it has the same uncompromisingly hard edge you hear in his productions. It’s a sound that won’t get lost in the mix! At the same time, it’s as responsive to input dynamics as any number of more conventional transistor distortion pedals.

Regarding parts: Using the exact original transistors matters little, IMHO, and I’d be leery of any product that uses their inclusion as the main selling point. Substituting other germanium/silicon pairs tended to sound at least as good as the original. Same with the diodes: Every diode type sounds a bit different, but there’s nothing particularly magical about the original germanium ones. I’d like to experiment more with silicon diodes and LEDs to see whether the add even more nasty edge.

This circuit is ripe for experimentation. I’m going to play with it more and see whether I can come up with an especially useful and exciting alternative — unless you beat me to it! 🙂

Percolator is my favorite fuzz. I have the Theremaniacs clone. I play single pickups only, and with both sliders halfway it is very dynamic, stacks greatly and cuts through the mix very well. At higher gain settings it can sound very synth-like as it is obvious in the video.

Its only drawback is noise, an audible hiss, which is a problem when you play softly.

Judging from the sound of the video, what comes out my speaker is closer to the almost-clone. I haven’t done any pedal building before but i’ve been tempted to try a clone as well, with various mods/and hmm… experiments!

3) Anyone have the slightest idea of “how it works”? This circuit could be a stumper for any aspiring electronics student. As far as I can tell it uses no known topologies for transistor circuits! Truly innovative!

4) How’s it sound on barre chords? You played mostly riffs and lead type things.

5) As always kudos for playing in a way that shows how the effect behaves, rather than demonstrating your awesome shredding prowess. I wish everyone else demoing effects on YouTube could pick up a hint.

Hmm — I haven’t tried, ’cause I don’t have any active guitars. It doesn’t have the fatal feedback resistor that makes Fuzz Faces so awful downstream from buffers, but I’d expect it to sound a little brighter and harsher when not “hearing” a passive pickup. But then, so does everything else. 🙂

The guitar is heavily loaded at the input — input impedance is 51K (feedback resistor from the base to emitter of the PNP)/100K (input pot)/some loss from the Ge transistor being biased quite low on the emitter (. It gets very bright and harsh after a buffer. You can build it with a switch that puts a 10K or 22K in series with the input and a 1nF cap to ground to correct the issue and imitate the input loading when using a buffer or active guitar, but it might ALSO be unhappy with the increased input.

Thanks for the info. I don’t really want to use it with an active guitar, it was just my sneaky way of getting an answer without giving away my swell idea (which will most likely take several years to realize).

Cool to see a side-by-side demo of a couple different versions of these to get a sense of what effect the changes have.

I think you’ll like experimenting with the diodes, especially because you said that you liked it best backed off a bit. The transistor distortion in this one is, I think far more magical than anything the diodes are doing. I don’t even find that the use of the resistor to get asymmetrical clipping to be that important — most of the magic is in the interaction of the two transistors, and raising the Fv of the diodes lets you hear more of it without going as far as removing them like in the Jerkulator.

Thanks, Jon. I agree that the diode portion is the circuit isn’t nearly as significant as the transistor portion. (As you point out, the Jerkulator doesn’t even have diodes.) To my ear, the diodes add a touch of compression and focus the lows, and the circuit sounds a bit better with them. But I didn’t even bother adding a diode on/off switch to the experimental version because the contrast just isn’t that interesting. I agree the circuit works pretty well with chords, though Jermaine is right — I didn’t play many in the demo. But there is a brief string of barre-chord triads at around 2:37 that’s quite representative.

And oh — this probably isn’t new info to some of you, but once again, this experiment reaffirms my belief that the part number of germanium transistors means next to nothing. Any two germanium transistors or equal hFE yield very similar results. Anyone who sells stompboxes based on the inclusion a specific model Ge transistor is probably full of it. There’s vastly more difference in sound between two Ge transistors of the same model and varying hFE (gain) value than there is between Ge transistors of different models but equal hFE.

Furthermore, much of the conventional wisdom about optimal hFE levels in vintage fuzz designs is just … wrong. That’s not to say that the commonly recommended hFE values don’t sound great — just that values that lie outside the ranges deemed necessary/optimal often sound fantastic. I wish more people relied on their ears rather than their multimeters.

that mojo shit is just that… bullshit. differences in tone are pretty minimal, gain range is way more important.

fwiw, in my experiments with this circuit (which led to the 4 knob juergulator) all ge sounds even better. on mine, i have a switch that chooses 2n404 or a simple 3906. gain is similar, but the ge is smoother.

better to socket and use your ears. people ask me what hfe i use in things, i say socket and experiment.. best way to go.

hi hiss in a perc is almost always from leakage… if anyone is having an issue, i’d try a different transistor.

to my ears, i get more tone/distortion with LOW gain, leaky as hell ge’s than the higher gains usually specified. higher gains tend to distort more, probably cuz it’s overdriving the second transistor more, giving more of a thick saturated compressed distortion… but hear me out… this often will NOT reach unity. but for some reason, lower gain ones will. my theory is that by not overdriving the second stage into clipping as hard, the sound isn’t as compressed and allows the second stage to amplify more rather than being strictly for more distortion.

ymmv… just my observations on this circuit. the juerg is interesting if ya gotta perc on the breadboard… the extra knobs can make a huge difference in the sound, depending on the other variables. with some things, they’re barely noticeable, with others they are ridiculously interactive.. depends on whether you’re fuzzing a clean amp or playing into some dirt..

On a very tangental note, the name of this pedal makes me think about how coffeemaking techniques have changed since the 70s. If it had been created in more recent times, would it have been dubbed the Harmonic French Press or the Harmonic Clover?

How about a vacuum coffee maker? My grandparents had one that sounded like a rocket ship taking off that coffee pot was around till the late 70’s? On the other hand my other granddad used a percolator to make caboose coffee. The caboose coffee took two or three times long percing and also had some salt in it (nasty stuff even old burnt 7-11 coffee is more drinkable). Caboose coffee would kill most starbuck customers in a single sip. I liked your take on the jerkulator with no diodes and the extra controls and all the controls sounded pretty useful. That being said I have been randomly tweaking controls on my pedal board lately (within reasonable limits) and then adjusting my playing till I find a good sound. It’s amazing to me how much “touch” can pull a good useable tone out.

Whether it generates more even order harmonics doesn’t affect the possibility of a “razor” sound. I interpret this as “increased higher harmonics” (whether even or odd). That depends on the sharpness of the corners of the waveform where it clips. So it could do both and still sound a lot different from tubes for just that reason. I don’t know whether it does or not. But I love the raw sound!

I just have to call out that “even order harmonics” thing because it gets misused too often. Yeah, you can measure it when you’re working with something pure, like a synthesized waveform. But a guitar’s sound is so complex that spectral analyses rarely reveal anything of the sort. Plus, the notion that even-order harmonics sound pleasant while odd-order ones sound nasty may be conventional wisdom, but it’s not especially wise. 😉

I have a recent issue 5 watt/EL84 amp (Gibson GA-5), and I’m actually in the process of tweaking some of the component values to tame the nasty overdrive & make it a bit more pedal friendly. I suspect that these amps were designed with the expectation that people wanted raunchier tones from them.

Not to cast aspersions on Mr. Albini, after all he’s well known for his musical and production exploits, while I’m not (sigh), but to hear him explain it, it seems like he’s not quite sure what it does and is just repeating what someone else told him.

You can take any signal into an audio editor and clip one side at zero to get the effect he is talking about. And it does not sound that distorted, even harmonics and all.

I haven’t seen the “Sardonic Albinator.” There are two versions of the original schematic floating around, with very slight differences. For my “origin” segment, I worked from those two, and when the disagreed, I just went with whichever option sounded best to me. For the mutated version, I changed a lot of stuff, and it doesn’t conform to any extant schematics. Hope that helps! 🙂

I remember first meeting Edwin "Ed" Geise in late 70's at an audio repair shop in Milwaukee, Wisconsin . I went there to have a home stereo amplifier repaired. I was very, very, young and Ed was a bit older than me, but he shared some of what he knew. I learned a lot from him (although I've forgotten too much over the years!!). I came in a few more times over the year and he showed me a way of measuring (leakage) in an audio circuit which at the time made no sense although I've forgotten too much over the years!!).

I was inspired by InterFax and remember they had the percolator, an equalizer in the same case as the percolator and cable testers and remember seeing the pedals in some local music stores and were fairly common up until early 90's. After InterFax closed I bought a percolator used from a local shop ( I think is was under $40) made around 20 clones of it around the year1986. I was in my late teens and only remember stores would not stock the clone as some still were selling used Interfax pedals. I still have parts from those clones.

What I do remember is Edwin spoke a lot about using non-audio frequencies as control signals embedded in audio tracks. I've seen a similar control concept used with AudioStrobe glasses. I think after InterFax closed he was selling electrical testers he designed from his house. I heard that Ed passed away a while ago in the early 90's.

I remember first meeting Edwin "Ed" Geise in late 70's at an audio repair shop in Milwaukee, Wisconsin . I went there to have a home stereo amplifier repaired. I was very, very, young and Ed was a bit older than me, but he shared some of what he knew. I learned a lot from him (although I've forgotten too much over the years!!). I came in a few more times over the year and he showed me a way of measuring (leakage) in an audio circuit which at the time made no sense although I've forgotten too much over the years!!).

I was inspired by InterFax and remember they had the percolator, an equalizer in the same case as the percolator and cable testers and remember seeing the pedals in some local music stores and were fairly common up until early 90's. After InterFax closed I bought a percolator used from a local shop ( I think is was under $40) made around 20 clones of it around the year1986. I was in my late teens and only remember stores would not stock the clone as some still were selling used Interfax pedals. I still have parts from those clones.

What I do remember is Edwin spoke a lot about using non-audio frequencies as control signals embedded in audio tracks. I've seen a similar control concept used with AudioStrobe glasses. I think after InterFax closed he was selling electrical testers he designed from his house. I heard that Ed passed away a while ago in the early 90's.

Your near-clone sounds more like my “stock” specs version built with the original transistor types and diodes. I agree with Pink Jimi, it’s all about the gain ranges.

Definitely try Jimi’s Juergulator if you get the chance; also, have a look at the “Albini” specs that the Mr. Bill mentioned in the Albini video shared at FSB. I built that on the Madbean board. I’m not sure it sounds exactly what Albini has in the video, it seems like more of a weird OD/distortion than a full-on fuzz. Very worth trying out as well…

while pulling q’s outta sockets looking for the “perfect” tone i discovered that you could pull one q while it was running, and it kept running… til the cap discharged. i was like… whaddafuggg?

further experimentation revealed you can run pretty much anything in this circuit. ge/si, all ge, or even all si. and it still sounds pretty much the same, partially cuz with the extra juerg knobs tweaking the bias points, you can really get on top of the tone. they’re kinda weird… they affect the tone less than the way it responds, and it takes a second to “catch up”…. sorta.

anyways, when all said and done, rev 2 is as close as google with video if ya wanna check it out, and if memory serves is all si this trip and sounds as good or better than the all ge ones i was building before.

Hey Jimi! Thanks for sharing yet another cool project. I’m still real fascinated with this circuit, and I’ve had a variation of the last one I play in the video on breadboard for months. (Don’t have the video pedal, though — I gave it to Matt Chamberlin, who, aside from being a brilliant rock star drummer, is also a guitar nut.) I confess I don’t really understand how or why the circuit works. But it’s sure fun to play with! (And oh — feel free to include links in comments. I’m totally cool with that.)

Hi!
I love the way you seem to really enjoy guitar -it’s childishly infectious! 😉
Anyhoo, is there a DIY/How To web page for the last experimental one in the vid?
I ke that one best… My 1st StompBox w my Fostex X-15 cassette 4 track with the I put fader all the way up!
I wanna make a clone that brings that sound back.

Thanks, Denis! That’s so nice to hear, becomes sometimes I feel SO old and jaded! I don’t have a project/plans for that experimental one. (I don’t even have the pedal — I gave it to Matt Chamberlain.) But it’s definitely one I plan to refine and , I hope, eventually release. In the meantime, Mad Bean makes a Percolator clone circuit board called the Pepper Spray: http://www.madbeanpedals.com/projects/PepperSpray/docs/PepperSpray.pdf

I found this site while researching to see if my home made homogenized harmonic percolator was doing what it’s supposed to or not. And i gotta say i love this site and your attitude towards guitar. Any shows planned for the N.Y.C. area?

[…] take on the classic circuit made popular by the rare and ever-elusive Interfax Harmonic Percolator, the Headache is ramped up to 11 and a half, for all the balls-out grit your amp and eardrums can […]

Once I can afford the transistors I plan on throwing one of these together 😉

What transistors are in the almost clone, and the last one? My ocd wants me to use the “correct” parts, but my ears just want something that sounds great. Oddly my bandmates prefer I don’t make things to facilitate crazy noise, but there’s no accounting for taste.

Made we wonder; why no DIY Club page about the percolator? I’ve really enjoyed your walkthroughs and I think this would make a really fun one.

Hiya Matthew! I don’t remember what I used in the almost clone. But in a way, I think that’s a good thing. After building clones countless clones of early stompboxes, I’ve found that almost without exception, there’s nothing magical or unique about the parts builders used back then. In almost every instance, builders just used whatever parts they had around, or whichever ones they could source most cheaply. It simply wasn’t an era of cork-sniffing.

I can see why someone might want to create an exact clone just for the satisfaction of it, but there’s no MUSICAL reason to do so. Especially since, once you start experimenting with different parts, it’s almost always possible to create something that something that sounds better that the original, at least to your ears. OCD can be a great quality in an instrument builder. My unsolicited (but friendly and sincere) advice would be to apply that quality to chasing the perfect parts for YOU, and not worry so much about what some tech plucked from a parts drawer in 1966.

I’m not sure how advanced a builder you are, but Tim Escobedo’s Harmonic Jerkulator variation sounds great and is relatively easy to make. If you stick with the parts in the schematic, they’ll only cost a couple of bucks.

I just built a Jerkulator, then added Si diodes. Somehow it sounds more like your Almost-Clone than it sounds like your Jerkulator. My theory is that this is due to my strat’s single-coil pickups being loaded differently than yours.

Ended up adding crossover distortion diodes before the diodes to ground … but this only sounds good with volume maxed. (I used Si, need to try Schottky or Ge.) Also tried Big Muff Pi tone control after everything. Seems to help with some playing styles (sounds very 90s) but really hurt others.

I woud like to ask if you have any schematics or information of what values did you use in your versions that you tested? Love almost a clone it sounds fast and full while the jerkulator is good but sound a bit thinner The last one sounds great but a bit too many controls 🙂

The almost clone used the same values as in the standard schematics you can easily find online. It simply didn’t use the hard-to-find transistors. My general belief is that much of the DIY vastly overstates the character and importance of particular transistors. Any number of combinations sound great. I don’t remember which ones I used here, and I gave the pedal away. Just get a few common silicon NPN and PNP transistors and pick whatever sounds best to you on breadboard! 🙂